Dopamine (DA) is a key modulator of motor and emotive pathways in the brain. Forebrain structures receive DA input exclusively from midbrain DA neurons, with cells of the substantia nigra pars compacta (SNc) projecting via the nigrostriatal pathway to dorsal striatum and those of the adjacent ventral tegmental area (VTA) project via the mesolimbic pathway to nucleus accumbens and other limbic structures. DA cells in both systems share common physiological properties, including somatodendritic release of DA. A significant difference, however, is that nigrostriatal DA cells degenerate in Parkinson's disease, whereas mesolimbic DA cells are spared. Several biochemical differences may contribute to greater SNc vulnerability, including weaker regulation of reactive oxygen species (ROS) in SNc than in VTA. This difference may be crucial, because oxidative stress has been proposed as a causal factor in Parkinson's disease. In addition to being potentially neurotoxic, however, ROS can act as signaling agents. Preliminary data implicate one ROS, hydrogen peroxide (H202), as a modulator of DA release. When applied exogenously, H202 inhibits release in the SNc and VTA as well as in dorsal striatum and nucleus accumbens. Additional data suggest that endogenous H202 generated during local stimulation inhibits DA release in the SNc and striatum, but not in the VTA. These data may reveal a normal physiological process in the nigrostriatal DA system that could contribute to oxidative stress, if regulation of H202 became disrupted. It is not clear, however, whether H202 per se is the modulator or whether it acts via related ROS. The goal of this R21 proposal is to determine specific ROS involved in modulating somatodendritic and synaptic DA release in the nigrostriatal vs. mesolimbic DA systems. Evoked DA release will be monitored using evoked using carbon-fiber microelectrodes and fast-scan cyclic voltammetry. Experiments in Aim t will compare regulation of somatodendritic DA release by ROS in the SNc and VTA, whereas those in Aim 2 will compare regulation in the dorsal striatum and the shell of the nucleus accumbens. Surprisingly, the studies proposed here would be among the first to investigate functional differences in ROS regulation between vulnerable and resistant DA systems. Consistent with the goals of the R21 program, these data should provide ground-breaking new information about underlying mechanisms in nigrostriatal degeneration.